1. Field of the Invention
The present invention relates to magneto-resistive elements that are widely used, for example, in magnetic random access memory (MRAM) used in data communication terminals, for example, and to manufacturing methods for the same.
2. Description of the Related Art
It is known that when a current flows through a multilayer film including ferromagnetic material/intermediate layer/ferromagnetic material in a direction traverse to the intermediate layer, a magneto-resistive effect occurs due to the spin tunneling effect if the intermediate layer is a tunneling insulating layer, and a magneto-resistive effect occurs due to the CPP (current perpendicular to the plane)-GMR effect if the intermediate layer is a conductive metal, such as Cu. Both magneto-resistive effects depend on the size of the angle between the magnetizations of the magnetic materials sandwiching the intermediate layers (magnetization displacement angle). In the former, the magneto-resistive effect occurs due to changes of the transition probability of tunneling electrons flowing through the two magnetic layers depending on the magnetization displacement angle, and in the latter the magneto-resistive effect occurs due to changes in the spin-dependent scattering.
When such a TMR element is used for a magnetic head or an MRAM device, one of the two magnetic layers sandwiching the intermediate layer can serve as a pinned magnetic layer, in which magnetization rotations with respect to an external field are difficult, by layering an antiferromagnetic material of FeMn or IrMn onto it, whereas the other layer serves as a free magnetic layer, in which magnetization rotations with respect to an external field are easy (spin-valve element).
When applying these vertical current-type resistive elements for example to a magnetic head or memory elements of an MRAM, for example in a reproduction element for tape media, then the area of the intermediate layer through which current flows should be not larger than several 1000 μm2, in order to achieve the demanded high recording densities or high installation densities. Especially in HDDs and MRAMs or the like, an element area of not more than several μm2 is desired. If the element area is large, magnetic domains form relatively easily in the free magnetic layer. Therefore, there are the problems of Barkhausen noise due to magnetic wall transitions when used as a reproduction element, and instabilities of the switching magnetization when used for the memory operation of MRAMs. On the other hand, in a region, in which the film thickness of the free magnetic layer with respect to the element area cannot be ignored, the demagnetizing field due to shape anisotropies becomes large, so that especially when used as a reproduction head, the decrease of the reproduction sensitivity brought about by an increase of the coercivity becomes a problem. When used as an MRAM, the increase of the reversal magnetic field becomes a problem.
In order to suppress the demagnetizing field, the film thickness of the free magnetic layer can be made thinner. However, at submicron dimensions, the film thickness of the magnetic layer necessary to suppress the demagnetizing field becomes less than 1 nm, which is below the physical film thickness limit of magnetic films.
Using the TMR elements for an MRAM, a thermal process at about 400° C. is performed in a semiconductor process of hydrogen sintering or a passivation process. However, it has been reported that in conventional pinned layers, in which IrMn or FeMn is arranged in contact with a magnetic layer, the MR is decreased by the decrease of the spin polarizability of the magnetic layer due to diffusion of Mn at temperatures of about 300° C. or above, and the decrease of the pinning magnetic field due to the dilution of the composition of the antiferromagnetic material (see S. Cardoso et. al., J. Appl. Phys. 87, 6058(2000)).
In previously proposed methods for reading non-volatile MRAM elements, the read-out is difficult when there are large variations in element resistance or in the resistance of switching element and electrode, because what is read out is the change of magnetic resistance of the magneto-resistive element with respect to the total resistance of magneto-resistive elements connected in series to a switching element and an electrode. In order to improve the S/N, a method of reading an element with the voltage between that element and a reference element has been proposed, but in that case, the higher integration of the elements becomes a problem, because the reference element is necessary (see p. 37, Proceedings of 112th Study Group of the Magnetics Society of Japan).